The evolution of neocortex in primates culminating in man is the most remarkable transformation in all of evolutionary history. In the traditional view the expansion of cortex was attributed to the increase in association areas. This conclusion has been reexamined over the past 15 years by the study of a graded series of species including opossum, hedgehog, tree shrew, and galago. These studies have shown that there is little or no association cortex as far up the scale as the prosimian. Instead the neocortex is composed chiefly of three extensive sensory fields: auditory, somatic and visual. The present proposal is aimed at uncovering the principles underlying the organization of these fields. At a first approximation it would seem that some principles are common to all fields. Each cortical field is the target of parallel pathways that differ in fiber size, relay in different thalamic nuclei, terminate in different layers of cortex, and to some extent terminate in different areas of the field. The aim of the research is to discover the functional significance of the various pathways by the behavioral deficit method. Because pathways overlap in a given cortical area, it is not easy to eliminate a pathway selectively by ablating a cortical area. Experiments are proposed in which separate paths are selectively destroyed. These studies will cast light on the role of the striate cortex and superior colliculus in vision. The evolution of cortex between the stages represented by prosimians and simians will be studied by adding the owl monkey to the series of species.